Method of laser casting copper-based composites

ABSTRACT

This invention presents the process of direct laser casting of copper alloys: Cu—X (where X=Ni, Fe, W;) and their composites Cu—Y and Cu—X—Y (Y=WC, TiC, Ti+C) from powders prepared using mechanical mixing and ball milling processes. Since the metallic powder is combined with a low melting point Cu metal, which has good thermal and electrical conductivity, the combination allows the powder mixture to be melted by CO 2  laser and re-solidified into a part with good mechanical properties and conductivity. The laser casting process for the Cu-based in-situ formation and the material systems formed using the said method have been developed. The process can be used to fabricate complex three-dimensional objects by multi-layer overlapping and the material systems can be used to build rapid tooling due to the properties of good thermal conductivity and low wear rate.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SG99/00116 which has an Internationalfiling date of Nov. 10, 1999, which designated the United States ofAmerica.

FIELD OF INVENTION

This invention relates to a method of laser casting, for example, acopper-based material. In particular, the invention presents a method todirectly melt pure copper (Cu) powder with the help of other elements(X) such as, for example, (nickel (Ni), iron (Fe) or tungsten (W)) usingCO₂ laser. Using this technique, Cu alloys (Cu+X) and composites Cu−Yand Cu−X−Y (Y=tungsten carbide (WC), titanium carbide (TiC), titanium(Ti) and graphite (C)) can be synthesised from elemental powder mixtureswhich are prepared by mechanical mixing or milling processes. Thedeveloped laser casting process may advantageously be used to fabricatecomplex three-dimensional objects, by multi-layer overlapping, which maybe used in electrical discharging machining (EDM) electrodes, rapid dieand mould tooling, or other system components.

BACKGROUND ART OF THE INVENTION

The method and apparatus of selective laser sintering (SLS) aredescribed in U.S. patents such as U.S. Pat. No. 4,863,538 (1989), U.S.Pat. Nos. 4,938,816 and 4,944,817 (1990), U.S. Pat. No. 5,076,869 (1991)and U.S. Pat. No. 5,182,170 (1993). In SLS, parts are built by selectivesintering or local melting of a binder in a thin layer of powderparticles using a CO₂ laser beam. The interaction of the laser beam withthe powder raises the temperature to the melting point of the powderbinder, resulting in particle bonding, fusing the particles to oneanother and to the previous layer. After an additional layer of powderis deposited via a roller mechanism on top of the sintered layer, thesucceeding layer is similarly sintered and built directly on top of it.In this way, the entire solid can be built layer by layer. Each layer ofthe building process consists of the required cross-section of the partat a given height. The unsintered powder in each layer remains in thepowder bed during processing to support overhangs and other structuresin subsequent layers. The completed part is revealed by brushing off theloose powder surrounding it and the unsintered powder can then bereused. Despite of the capability of the SLS to build parts of variousmaterials, post-processing, such as debinder and Cu infiltration, isoften needed to achieve working strength. Shrinkage of the built partafter the debinder and infiltration process results in distortion.

A selective metal powder sintering process was described by Van derSchueren and Druth in “Powder deposition in selective metal powdersintering” in Rapid Prototyping Journal, Vol. 1, Number 3, 1995,pp23-31. In this process particles in a Fe—Cu powder mixture wereselectively bound by means of liquid phase sintering initiated by aNd-YAG laser beam. The powder deposition mainly depended on the powderproperties—in this case on the individual Cu or Fe powder properties—andresulted in compromises on the powder mixtures as well as inmodifications of the deposition mechanism.

EOSINT M system, as described in U.S. Pat. Nos. 5,753,274, 5,730,925,5,658,412, was the first commercial system for direct laser sintering ofmetallic powder. The word “direct” implies that the materialconstituents are directly laser sintered to produce a high density partrequiring little or no post-processing. A related patent on parts formedby direct sintering is U.S. Pat. No. 5,732,323 which describesprocessing of powders based on an iron-group metal. Currently, the onlymetallic material that is available commercially for direct metalsintering is a bronze-nickel alloy by Electrolux and a newly developedmetal powder M Cu 3201 by EOS. Direct selective laser sintering involvesdirectly melting and consolidating selected regions of a powder bed toform a desired shape having high or full density. Direct metal lasersintering involves melting the component matrix and obtaining theappropriate amount of flow from the molten material. The appropriateamount of flow is critical and can be described as the flow thateliminates porosity, produces a highly dense part and maintains tightdimensional tolerances. The appropriate amount of flow is controlled byfactors such as atmosphere, powder bed temperature and laser' energydensity. Three important parameters governing the energy density arelaser power, scan spacing and scan speed²:

A_(n)=P/νδ(J/cm²)  (1)

where A_(n) is the energy density; P is the incident laser power(Watts); ν is the laser scan speed (cm/s); and δ is the scan spacing(cm).

If the energy density is too high, the surface begins to vaporize beforea significant depth of molten material is produced. The sintered layerthickness decreases with increasing scan speed due to the shorterinteraction (sintering) time. This thickness also decreases withdecreasing scan line spacing if the laser beam spot is larger than thespacing. More scan overlapping will occur with smaller scan linespacing. The thermal conductivity and reflectivity of the sintered solidare higher than those of the powder. When more scan overlapping occurs,more laser energy will be transferred away by heat conduction throughthe sintered solid and reflected away by the sintered solid surfaceresulting in a decrease in the layer thickness.

The amount of light energy of the laser beam absorbed by a metallicsurface is proportional to 1-R, where R is its reflectivity. Thereflectivity of a material is defined as the ratio of the radiant powerreflected to the radiant power incident on the surface. It indicates thefraction of the incident light that is absorbed and contributes toheating effects, and is most dependent on the electrical conductivity. Ametal with high electrical conductivity has high reflectivity, forexample, copper and nickel. High-density energy is required to sinter amaterial with high reflectivity, such as Cu. Another importantcharacteristic is thermal diffusibility. A material with high thermaldiffusibility will normally allow a greater depth of fusion penetrationwith no thermal shock or cracking.

At the CO₂ laser wavelength of 10.6 μm, where R is close to unity, 1-Rbecomes very small. High-density energy is thus required to sinter amaterial like copper. The difference in the value of R becomes importantat long wavelengths. For copper at 10.6 μm, 1-R is about 0.02, whereasfor steel it is about 0.05. As steel absorbs 2.5 times as much of theincident light as copper, it is easier to melt steel with a CO₂ laserthan metals such as aluminum or copper. Attempts to coat the powdersurface to improve heat absorption or reduce reflection are not alwayseffective because of poor thermal coupling between the coating and thepowder. The reflectivity problem has been a barrier to the applicationof CO₂ lasers to the melting of metals such as copper or gold.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a method oflaser casting a metal-based alloy or composite comprising:

milling elemental metal powder having a relatively high reflectivity atthe wavelength of the laser with at least one other material whichabsorbs laser energy more readily than said elemental metal powder toform said metal-based mixture; and

laser casting said metal-based mixture; wherein said milling isconducted for a period sufficient to form at least a partial coating ofsaid at least one material on particles of said elemental metal powder.

More particularly the invention provides a method of laser casting acopper-based alloy or composite comprising:

milling elemental copper powder with at least one other material whichabsorbs laser energy more readily than elemental copper powder to formsaid copper-based mixture; and

laser casting said copper-based alloy or composite by application of alaser to said copper-based mixture; wherein said milling is conductedfor a period sufficient to form at least a partial coating of said atleast one material on particles of said elemental copper powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates a process of laser casting in building strips;

FIG. 1b illustrates the process of laser casting in a multi-layeredpart.

FIG. 2 shows an x-ray diffraction (XRD) results of Cu-Ni powder beforeand after the laser casting process.

FIG. 3a shows the microstructure of the laser cast Cu-Ni material.

FIG. 3b shows a laser-cast tracking having smooth surface morphology.

FIG. 4 shows the x-ray (XRD) diffraction spectra of a Cu-WC(Ni) systembefore and after laser casting.

FIG. 5a shows the microstructure of the laser cast Cu-6.7% WC composite;

FIG. 5b is a photomicrograph showing the segregation of WC particles;

FIG. 5c shows the cross-section of a multi-layered part with CU 19.13%Ni 7.5% WC.

FIG. 6 shows an x-ray diffraction (CRD) spectrum of a CU-Ti-C(Ni) systemcomparing mixture of powders to after laser casting.

FIG. 7a shows the dense microstructure of the Cu-Ti-C-Ni system.

FIG. 7b is another view of the microstructure of the Cu-Ti-C-Ni system.

FIG. 8 shows the x-ray diffraction (XRD) spectrum of laser cast Cu 10%Fe.

DETAILED DESCRIPTION

The following descrip tion of the invention will be limited to Cu basedsystems. However, it will be recognised that the principle of theinvention may also be applied to other metal systems where the lasercasting of a metal having high reflectivity is required.

Cu is a very versatile and common material for applications requiringgood electrical and thermal conductivities, especially for use as a basematerial in EDM electrodes. However, Cu is not easily melted by CO₂laser due to its high reflectivity to the laser. The present inventionin one aspect advantageously provides a method of melting Cu using CO₂laser with the help of other element(s) to form Cu alloys and Cu-basedin-situ composites.

Elemental Cu powder is initially milled with other elemental powder orcompounds which absorb laser energy more readily. To achieve a certaindegree of coating of the second element on the Cu particles, the powdermixture is preferably ball milled for about 1 to 2 hours under theprotection of Argon gas. The mills may be planetary ball mills,attritors and horizontal mills. Relatively high energy of milling isused. The coating on the Cu particles advantageously enhances theconduction of heat to the adjacent Cu particles. 0 to 3% by weight ofprocess controlling agent (PCA), which can be, for example, stearic acidor other low melting organics and flux, may be used to prevent orminimize cold welding. If graphite is used, no PCA should be introducedin the milling. In general, minimal PCA should be incorporated into thepowder mixture to reduce contamination.

Besides material compositions which will be discussed hereafter, theproperties and quality of the part may be altered by altering theprocessing parameters, such as laser power output, beam spot size andits scanning speed. As such, these parameters are advantageouslycontrolled in order to achieve optimal performance. In forming theparticular material system and building the multi-layered parts, theprocessing parameters are preferably controlled to provide a laser powerof 50-1000 W, beam spot size of 0.2-5.0 mm and laser scanning speed of100-1500 mm/min.

In the laser casting process, inert gas such as Argon, is used toprevent oxidation of materials to be cast. When Ti is used, inert gasprotection becomes more important. A reduction atmosphere of CO may alsobe used.

All powder systems are preferably mechanically mixed for at least aboutone hour followed by ball milling for at least about one to two hours toachieve at least partial coating of Ni, W, Fe, Ti, TiC or WC on the Cuparticles. The ball mill machine is preferably run at a speed of 150-300rpm using a ball size of 15-30 mm diameter with weight ratio ofball-to-particle-size of from 5-20:1 for a duration of from 1 to 4hours. Up to 3% by weight of process controlling agent (PCA) may beadded in the ball milling to prevent excessive cold welding. If C isused during milling, PCA should not be added to reduce surfacecontamination during milling.

Laser scanning speed preferably spans from 100 to 1500 mm/min using50-1000 W laser power. Beam spot size generally ranges from 0.1-5.0 mm.These parameters should provide sufficient heat energy to melt the Cucomponent well. However, to build a fine layer part, the spot size canbe further reduced and scanning speed increased. Inert gas of Argon isadvantageously used during the laser scan. Due to the high reactivity ofTi with oxygen, an Argon chamber with purity of at least 99% ispreferably used.

To obtain optimal parameters for a particular powder mixture, astainless steel plate of approximately 5 mm thickness may be placed on aflat surface as a substrate. A 1 mm thick stainless steel plate with acentral cut out is advantageously placed on top of a thicker plate.Metal powder is then preferably laid in the frame and lightly compressedand flattened by a roller in order to form a uniform powder bed. Thelaser beam is then programmed to scan the powder bed in horizontalstrips, with each strip formed at a different laser scanning speed.

FIG. 1 illustrates the process of laser casting in building strips (FIG.1a) and a multi-layered part (FIG. 1b). After the first layer of powderis scanned, the “piston” like part is lowered down and another layer ofpowder is spread on top of the preceding layer of powder. The unevenpowder is slightly compressed and levelled using the levelling roller.The new layer of powder is then scanned by the laser. This process isrepeated until a three-dimensional part is built.

In the laser scanning process of a Cu—Ni system, chemical homogenisationbetween the Cu and Ni occurs to form a homogeneous melt. The newhomogeneous solid phase thus formed is Cu—Ni. The amount of Ni in thesolid phase is preferably at least 5%. Even using a low percentage ofNi, the powder may still be laser-cast to form a dense part. A study ofmicrostructure of the Ni—Cu system reveals a dendritic structure. It hasbeen determined that as the amount of Ni is decreased from 57.5% to9.59%, the dendritic microstructure progressively disappears.

In Cu—Ti—C, Cu—Ni/Ti—C systems, Cu and a mixture of Cu and Ni are usedas the matrix while Ti and C react with each other to form in-situ TiCas reinforcement. The formation of fine hard TiC particles increases thestrength, Young's modulus and wear resistance of the part produced. Themaximum volume percentage of TiC is generally 50%.

In Cu—Ni/TiC systems, TiC is incorporated into a Cu—Ni matrix by lasercasting the Cu—Ni together with TiC particles. It has been observed thatthe TiC is uniformly distributed in the Cu—Ni solid solution. Theresulting microstructure shows good interface between the reinforcementphase and the matrix phase.

Cu/WC and Cu—Ni/WC composite parts may be synthesised using Cu/WC,Cu—Ni/WC systems. Cu can be melted with or without the help of Ni. WCmay also be used to facilitate heat absorption and the melting of Cu.Since WC is much heavier than Cu and Cu—Ni solution, WC particlesusually settle down to the bottom of the laser scanned line or strip. Tominimize or prevent inhomogeneity in this case, thin laser scan linesare advantageously used.

Cu—Fe systems are also available using the present method. In this caseat least 10% of Fe is preferable used for successful laser casting. MoreFe may enhance formability of parts but will decrease electrical andthermal conductivities. Depending on applications, the amount of Fe maybe varied from 10% to 50%.

When considering a Cu—W (Ni) system, about 10% W is preferably used toensure successful laser casting. Since Cu and W are not solutable, Cumay be pushed to the two sides of the laser cast melt line. A thin lasercast line may help to reduce inhomogeneity of distribution of Cu in thiscase. Addition of Ni advantageously increases the wetting between thethree constituents.

EXAMPLES

The following examples illustrate embodiments of the invention only andshould not be construed as limiting on the invention in any way.

Example 1

Cu—Ni system

For the Cu and Ni powder system, the percentage of Ni was varied from10% to 67% by weight. The powder mixture was first ball milled to form apowder mixture. After ball milling, laser casting was carried out.Formation of Cu—Ni solid solution was observed. Even at low percentagesof Ni, the powder mixture could still be melted and formed into a densepart. The Cu—Ni powder system was therefore used as the matrix for Cualloy based composites. FIGS. 2a and 2 b show the X-ray diffraction(XRD) analysis results of the Cu—Ni powder system before and after thelaser casting process. FIG. 2a shows the as-received Cu—Ni XRD spectrumwhere Cu and Ni diffraction peaks can be seen. After laser casting, pureCu and Ni peaks disappeared and a new solid solution of Cu—Ni phase wasdetected. Microstructure of the laser cast Cu—Ni material is shown inFIG. 3a. Even at low percentage of Ni, a homogeneous phase has beenformed without porosity. As can be seen in FIG. 3b, the laser-cast trackhas a smooth surface morphology.

Example 2

Cu—WC (Ni) system

Cu was first ball milled with WC (with small amount of W₂C in the WCpowder). Between 10% to 60% by weight of WC was used in the process.Structural analyses using X-ray diffraction before and after lasercasting are shown in FIGS. 4a and 4 b. X-ray diffraction spectra show nostructural change after laser casting. Microstructure studies indicateporosity in the laser cast part when the powder mixture contains lessthan about 10% by weight of WC. FIG. 5a shows the microstructure of thelaser cast Cu-6.7% WC composite. Porosity in the cast part can be seenin the figure. With increasing percentage of WC, porosity can bereduced. With the addition of Ni into the powder system (Cu—Ni matrix),the porosity was eliminated and a highly dense part of WC particlesembedded in a solid solution of nickel in copper was formed. Ni additioncan increase bonding between Cu and WC. The microphotograph in FIG. 5bshows that WC particles segregate at the boundaries of the cast layers.During laser casting, the heavy WC particles sank to the bottomresulting in the segregation of the WC particles. FIG. 5c shows thecross section of a multi-layered part with Cu19.13% Ni7.5% WC. Themicrostructure reveals high density with no porosity after the additionof elemental Ni. The boundary between layers has slightly less WC due toremelting of the previous layer.

Example 3

Cu—Ti—C (Ni) system

Laser casting of Cu—Ti—C (Ni) should be well protected to avoidoxidation of Ti powder since it is very reactive. Besides the use of anArgon chamber, direct purging with Argon gas is also possible. In-situformation of TiC was observed in Cu—Ti—C systems as shown in FIG. 6. Theformation of in-situ TiC can be observed from the TiC diffraction peaks,as shown in FIG. 6b. After the laser scan, Ti and C reacted completelywith each other and no Ti was detected from the XRD results.Microstructure analysis showed that TiC was well distributed in thematrix. However, the part formed after laser casting was found to berelatively porous. At high percentages of Ti and C (calculated resultant50% TiC) in the powder mixture of Cu—Ti—C, the powder did not fusetogether after the laser sintering process. With the addition of Ni (atabout 10%), the powder fused together to form a part with low porosity.FIG. 7 shows the dense microstructure of the Cu—Ti—C—Ni system.

Example 4

Cu—Ni—TiC system

Cu, Ni and TiC were ball milled to form a mixture. After ball milling,TiC particles were in general embedded in a solid solution of Cu in Nimatrix. Laser casting was carried out using the method as describedabove. Good particle-matrix interface after casting was observed. UnlikeCu—WC system, no segregation of TiC was observed. The multi-layeredstructure obtained showed a homogeneous distribution of TiC in Cu—Nimatrix.

Example 5

Cu—Fe

Cu and Fe can be co-melted. The minimum amount of Fe used for the laserscan should be at least about 10%. If Fe content is less than 10%, itmay lead to a porous structure and agglomeration of Cu. FIG. 8 shows theXRD spectrum of the laser cast Cu10% Fe. Although Cu—Fe system does notform compound, they are well mixed. Less than 10% Fe may causedifficulty in the melting of Cu and cause of agglomeration of Cu melts.Slight amount of Ni may help to increase wetability.

What is claimed is:
 1. A method of laser casting a copper-based alloy orcomposite comprising: milling elemental copper powder with at least oneother material which absorbs laser energy more readily than elementalcopper powder to form a copper-based mixture; and laser casting saidcopper-based alloy or composite by application of a laser to saidcopper-based mixture; wherein said milling is conducted for a periodsufficient to form at least a partial coating of said at least onematerial on particles of said elemental copper powder.
 2. The methodaccording to claim 1, wherein said milling step comprises ball millingsaid powder mixture for a period of from about 1 to 4 hours.
 3. Themethod according to claim 1, wherein said milling step comprisesmechanically mixing said powder mixture for a period of at least about 1hour and subsequently ball milling said powder mixture for a period ofabout 2 hours.
 4. The method according to claim 2, wherein said ballmill is run at a speed of from about 150 to about 300 rpm using a ballsize of from about 15 mm to about 30 mm diameter.
 5. The methodaccording to claim 2, wherein the weight ratio of ball to powder is from5-20:1.
 6. The method according to claim 1, wherein said at least oneother material comprises elemental Ni.
 7. The method according to claim6, wherein said Ni is present in an amount of at least about 5% byweight.
 8. The method according to claim 1, wherein said at least oneother material comprises elemental Ti and C.
 9. The method according toclaim 8, wherein said at least one other material further compriseselemental Ni.
 10. The method according to claim 8, wherein the amount ofTi and C in said mixture are such that during said laser casting step,said Ti and C react with each other to form in situ TiC in an amount of50% by weight or less, and wherein said Ni is present in an amount ofabout 10% by weight.
 11. The method according to claim 1, wherein saidat least one other material comprises TiC and elemental Ni.
 12. Themethod according to claim 1, wherein said at least one other materialcomprises WC.
 13. The method according to claim 12, wherein said WC ispresent in an amount of from about 10% to 60% by weight.
 14. The methodaccording to claim 12, wherein said at least one other material furthercomprises elemental Ni.
 15. The method according to claim 12, whereinsaid at least one other material further comprises elemental W.
 16. Themethod according to claim 1, wherein said at least one other materialcomprises elemental Fe.
 17. The method according to claim 16, whereinsaid Fe is present in an amount of at least about 10% by weight.
 18. Themethod according to claim 17, wherein said Fe is present in an amount offrom about 10% to 50% by weight.
 19. The method according to claim 16,wherein said at least one other material further comprises Ni.
 20. Themethod according to claim 1, wherein said at least one other materialcomprises elemental W or Ni.
 21. The method according to claim 20,wherein said W is present in an amount of about 10% by weight.
 22. Themethod according to claim 1, wherein process controlling agent is addedin said milling step in an amount to substantially prevent cold weldingof particles of said mixture.
 23. The method according to claim 22,wherein said process controlling agent is added in an amount of up toabout 3% by weight.
 24. The method according to claim 1, wherein thelaser applied to said mixture during said laser casting is a CO₂ laserwith a wavelength of 10.6 μm.
 25. The method according to claim 24,wherein said laser is selectively applied to said mixture at a laserscanning speed of from 100 to 1500 mm/min.
 26. The method according toclaim 24, wherein said laser is applied at a laser power of from about100 to about 1500 W using a beam spot diameter of from about 0.2 toabout 5.0 mm.
 27. The method according to claim 1, wherein said lasercasting step is carried out under an inert atmosphere of argon gas. 28.The method according to claim 1, wherein said laser casting is carriedout under a 10 reduction atmosphere of CO.
 29. An article cast using amethod according to claim
 1. 30. The article according to claim 29,comprising a single-layer or multi-layer structure.
 31. The articleaccording to claim 29, wherein said article is an EDM electrode, rapiddie and moulding tooling or system composites.
 32. A method of lasercasting a metal-based alloy or composite comprising: milling elementalmetal powder having a relatively high reflectivity at a wavelength ofthe laser with at least one other material which absorbs laser energymore readily than said elemental metal powder to form a metal-basedmixture; and laser casting said metal-based allay or composite byapplication of a laser to said copper-based mixture; wherein saidmilling is conducted for a period sufficient to form at least a partialcoating of said at least one material on particles of said elementalmetal powder.